The laser diode has been used frequently as a light source in a variety of photographic applications. For example, in the field of laser printing, a laser beam is focused through a lens and then scanned onto a photosensitive medium, such as film. The laser diode is often controlled by digital data stored in a computer. Image quality may be improved by forming an image from pixels having varying gray tones to form a continuous tone image in film. The continuous tone radiographic laser printer is one application in which a laser diode is used to expose radiographic film to electronic images produced by medical imaging modalities, such as computed tomography, magnetic resonance imaging, digital subtraction angiography, ultrasonic imaging, and the like.
A problem with laser diodes is that the illuminative output of a semiconductor laser diode is not linear across its entire operating range. More specifically, as shown in FIG. 11 (which is a typical graph of light output P of a laser diode as a function of current I), the resulting curve has a lower level non-linear operating region joined by a knee region to a higher level operating region where light output varies linearly as a function of input signal value. The linear region is known as the lasing region and the non-linear lower level region is known as the light emitting or spontaneous emission region. The curve includes (1) a lower portion between 0 and the knee current I.sub.k where the resulting optical power output P.sub.d varies in a non-linear manner as current I.sub.d changes, and (2) a second higher portion between I.sub.k and I.sub.max where optical power output varies linearly with changes in current I.sub.d. Because the non-linear region is unsatisfactory for generating a continuous tone image in response to image input signal levels, it is desirable that the laser output range be limited to the linear operating region. It is thus common practice to operate a laser diode at or above a predetermined current level called a threshold current.
Problems arise in automatically setting and maintaining a constant laser output power. Other problems arise in protecting the laser diode from overpower, over and under temperature and transient voltage or current spikes on power up and down conditions. U.S. Pat. No. 4,507,767, issued Mar. 26, 1983, inventor Takasugi, discloses an optical disk device including a laser diode as a light source which includes a protection circuit in parallel with the semi-conductor laser, in which the protection circuit is put in a short-circuited state between two terminals thereof, when a monitor signal corresponding to the light output of the semi-conductor laser exceeds a predetermined value. There is no disclosure in this patent of protection of the semi-conductor laser diode due to under or over temperature conditions, or power on and power off voltage and current spikes.
U.S. Pat. No. 4,872,080, issued Oct. 3, 1989, inventors Hentschel et al., discloses a protective circuit for a semi-conductor laser supplied with an excitation current by a current generator circuit. The protective circuit includes a transducer whose output is controlled by the laser light, a limit circuit cooperating with the transducer and a current shunt circuit energized by the limit circuit and switched in parallel with the semi-conductor laser. Once the permissible emitted light power is reached, excessive excitation current is diverted to the shunt circuit. There is no disclosure in this patent of protecting the laser diode from over or under temperature conditions, among other problems.
U.S. Pat. No. 4,791,636, issued Dec. 13, 1988, inventors Yamamoto et al, discloses a semi-conductor laser comprising a laser oscillating optical waveguide composed of a control region which functions to absorb light and main regions which function to oscillate laser light, wherein the laser device includes a shunting means by which the ratio of the current flowing to the control region to the total current injected into the laser device is set according to a predetermined algorithm.
U.S. Pat. No. 4,074,334, issued Feb. 14, 1978, inventors D'Arrigo, et al., discloses a protective device for limiting the current flow through a power transistor, a normally nonconductive shunt transistor is turned on to shunt current around the power transistor when collector current and collector-emitter voltage exceed a selected current/voltage curve. There is no monitoring disclosed in this patent of over or under temperature conditions.
The following U.S. Patents disclose various laser diode control techniques which do not sufficiently solve the problems of automatic power control and protection of a laser diode: U.S. Pat. No. 5,019,769, issued May 28, 1991, inventor Levinson; U.S. Statutory Invention Registration No. H322, published Aug. 4, 1987, inventor Simons; U.S. Pat. No. 4,890,288, issued Dec. 26, 1989, inventors Inuyama et al; U.S. Pat. No. 4,501,022, issued Feb. 19, 1985, inventor Oswald; and U.S. Pat. No. 4,612,671, issued Sept. 16, 1986, inventor Giles.